Field of the Invention
This invention relates to high strength, high toughness steel alloys, and in particular, to such an alloy that provides a unique combination of tensile strength and toughness when hardened and tempered.
Description of the Related Art
Age-hardenable martensitic steels that provide a combination of very high strength and toughness are known. Among the known steels are those described in U.S. Pat. No. 4,076,525 and U.S. Pat. No. 5,087,415. The former is known as AF1410 alloy and the latter is sold under the registered trademark AERMET. The combination of very high strength and toughness provided by those alloys is a result of their compositions which include significant amounts of nickel, cobalt, and molybdenum, elements that are typically among the most expensive alloying elements available. Consequently, those steels are sold at a significant premium compared to other alloys that do not contain such elements.
More recently, a steel alloy has been developed that provides a combination of high strength and toughness comparable to the AERMET and AF1410 alloys, but without the need for cobalt and with significantly lower amounts of nickel and molybdenum than those alloys. One such steel is described in U.S. Patent Application Publication No. 2011/0165011. The steel described in that patent is an air hardening SiCuNiCr steel alloy. The steel described in the '011 application is capable of providing combinations of very high strength and toughness even when tempered at about 500° F. For example, longitudinal specimens of one embodiment described in the '011 application provided a tensile strength of at least 290 ksi in combination with a Charpy V-notch (CVN) impact strength of at least 20 ft-lbs in the hardened and tempered condition. Longitudinal specimens of another embodiment provided a tensile strength of at least 310 ksi in combination with a CVN impact strength of at least about 16 ft-lbs in the hardened and tempered condition.
However, the potential use of such steels in critical aerospace components has driven a need to extend the combination of strength and toughness provided by such alloys to higher levels than previously achieved. Consequently, a need has arisen for an air-hardening SiCuNiCr steel alloy that provides tensile strength in excess of about 295 ksi in combination with an impact toughness in excess of about 15 ft-lbs. This combination of properties should be provided after the alloy has been tempered at about at least 500° F. Since it is known that toughness and tensile strength are inversely related, the solution to this need is not easily achieved.